Conventionally, a discharge lamp has been used that employs fluorescent substance as a light source. Among the discharge lamps, a cold-cathode type discharge lamp has been used as a backlight for a liquid crystal display (LCD) because a diameter of its glass tube can be reduced.
The cold-cathode type discharge lamp has a configuration in which its glass tube is equipped with electrodes at its opposed ends, a rare gas such as Argon and mercury are enclosed in an inside space of the glass tube, and fluorescent substance is coated into an interior of the glass tube.
FIG. 1 is a cross-sectional view of important components of a configuration of a conventional cold-cathode type discharge lamp. The discharge lamp 51 is equipped with an electrode 53 at each of the two opposed ends of its glass tube 52. A rare gas such as argon gas and mercury are enclosed in an inside space of the glass tube 52 and any fluorescent substance 52a is coated to a predetermined region in an interior of the glass tube 52.
The electrode 53 has a cup 54. The cup 54 has such a shape that a bottom is provided therein and one end thereof is open, and the cup 54 is connected to a forward end of a lead-in wire 55 which is inserted through an end of the glass tube 52 and held in position thereby.
The light emission principle of the cold-cathode type discharge lamp 51 will be explained as follows: when a voltage is applied between the electrodes 53 at a high frequency, glow discharge occurs so as to emit electrons from the cup 54. The electrons emitted from the cup 54 are accelerated, thereby colliding with mercury atoms in such a way as to excite them. The mercury atoms thus excited emit ultraviolet light. This ultraviolet light is converted into a visible light by the fluorescent substance 52a, thereby rendering the discharge lamp 51 luminiferous.
The conventional cold-cathode type discharge lamps face a problem such that a large drop in cathode voltage occurs during operations. In other words, it faces a problem such that a large quantity of power can be dissipated by the electrodes themselves but fails to contribute to light emission, thus resulting in a low luminous efficiency relative to dissipation power.
Further, they suffer from such a problem that so-called ion sputtering in which any ions generated during discharge collide with electrodes and so waste then occurs to a conspicuous degree. As the cup wastes away, it becomes incapable of emitting a sufficient quantity of electrons, thus resulting in a diminution in the luminance. This brings about a problem of a shortened service life of the electrodes. Such the shortened service life of the electrodes in turn results in a limited service life of the discharge lamp.